On Tue, 26 Apr 2011 00:19:46 -0700 Ying Han <yinghan@xxxxxxxxxx> wrote: > On Mon, Apr 25, 2011 at 6:38 PM, KAMEZAWA Hiroyuki > <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote: > > On Mon, 25 Apr 2011 15:21:21 -0700 > > Ying Han <yinghan@xxxxxxxxxx> wrote: > >> Thank you for putting time on implementing the patch. I think it is > >> definitely a good idea to have the two alternatives on the table since > >> people has asked the questions. Before going down to the track, i have > >> thought about the two approaches and also discussed with Greg and Hugh > >> (cc-ed), Âi would like to clarify some of the pros and cons on both > >> approaches. ÂIn general, I think the workqueue is not the right answer > >> for this purpose. > >> > >> The thread-pool model > >> Pros: > >> 1. there is no isolation between memcg background reclaim, since the > >> memcg threads are shared. That isolation including all the resources > >> that the per-memcg background reclaim will need to access, like cpu > >> time. One thing we are missing for the shared worker model is the > >> individual cpu scheduling ability. We need the ability to isolate and > >> count the resource assumption per memcg, and including how much > >> cputime and where to run the per-memcg kswapd thread. > >> > > > > IIUC, new threads for workqueue will be created if necessary in automatic. > > > I read your patches today, but i might missed some details while I was > reading it. I will read them through tomorrow. > Thank you. > The question I was wondering here is > 1. how to do cpu cgroup limit per-memcg including the kswapd time. I'd like to add some limitation based on elapsed time. For example, only allow to run 10ms within 1sec. It's a background job should be limited. Or, simply adds static delay per memcg at queue_delayed_work(). Then, the user can limit scan/sec. But what I wonder now is what is the good interface....msec/sec ? scan/sec, free/sec ? etc... > 2. how to do numa awareness cpu scheduling if i want to do cpumask on > the memcg-kswapd close to the numa node where all the pages of the > memcg allocated. > > I guess the second one should have been covered. If not, it shouldn't > be a big effort to fix that. And any suggestions on the first one. > Interesting. If we use WQ_CPU_INTENSIVE + queue_work_on() instead of WQ_UNBOUND, we can control which cpu to do jobs. "The default cpu" to run wmark-reclaim can by calculated by css_id(&mem->css) % num_online_cpus() or some round robin at memcg creation. Anyway, we'll need to use WQ_CPU_INTENSIVE. It may give us good result than WQ_UNBOUND... Adding an interface for limiting cpu is...hmm. per memcg ? or as the generic memcg param ? It will a memcg parameter not a threads's. > > > >> 4. the kswapd threads are created and destroyed dynamically. are we > >> talking about allocating 8k of stack for kswapd when we are under > >> memory pressure? In the other case, all the memory are preallocated. > >> > > > > I think workqueue is there for avoiding 'making kthread dynamically'. > > We can save much codes. > > So right now, the workqueue is configured as unbounded. which means > the worse case we might create > the same number of workers as the number of memcgs. ( if each memcg > takes long time to do the reclaim). So this might not be a problem, > but I would like to confirm. > >From documenation, max_active unbound workqueue (default) is == Currently, for a bound wq, the maximum limit for @max_active is 512 and the default value used when 0 is specified is 256. For an unbound wq, the limit is higher of 512 and 4 * num_possible_cpus(). These values are chosen sufficiently high such that they are not the limiting factor while providing protection in runaway cases. == 512 ? If wmark-reclaim burns cpu (and get rechedule), new kthread will be created. > > > >> 5. the workqueue is scary and might introduce issues sooner or later. > >> Also, why we think the background reclaim fits into the workqueue > >> model, and be more specific, how that share the same logic of other > >> parts of the system using workqueue. > >> > > > > Ok, with using workqueue. > > > > Â1. The number of threads can be changed dynamically with regard to system > > Â Â workload without adding any codes. workqueue is for this kind of > > Â Â background jobs. gcwq has a hooks to scheduler and it works well. > > Â Â With per-memcg thread model, we'll never be able to do such. > > > > Â2. We can avoid having unncessary threads. > > Â Â If it sleeps most of time, why we need to keep it ? No, it's unnecessary. > > Â Â It should be on-demand. freezer() etc need to stop all threads and > > Â Â thousands of sleeping threads will be harmful. > > Â Â You can see how 'ps -elf' gets slow when the number of threads increases. > > In general, i am not strongly against the workqueue but trying to > understand the procs and cons between the two approaches. The first > one is definitely simpler and more straight-forward, and I was > suggesting to start with something simple and improve it later if we > see problems. But I will read your path through tomorrow and also > willing to see comments from others. > > Thank you for the efforts! > you, too. Anyway, get_scan_count() seems to be a big problem and I'll cut out it as independent patch. Thanks, -Kame -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . 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